This invention relates to the control of the thrusters of a spacecraft, and more particularly, to attitude and velocity control by the use of throttleable or pulse-modulated thrusters.
Spacecraft are widely used for communications, earth sensing and exploration, vehicle locating, and for surveillance. All of these uses require that the orientation of the spacecraft in space, and possibly its location, be accurately controlled. The spacecraft attitude may be controlled by magnetic torquers, by momentum or reaction wheels, or by the use of thrusters.
Since each spacecraft at launch has a slightly different configuration than other spacecraft, its thrusters will be placed at slightly different locations relative to the center of gravity than in other spacecraft. Also, the specific impulse characteristics of the thrusters may differ from one to the next. As a consequence, the logic for controlling the thruster system is ordinarily custom-written for each spacecraft. During the construction phase for a spacecraft, it may be found to be necessary to alter the configuration or thruster characteristics, which may require costly rewriting of software.
Control arrangements ordinarily provide torques by preselected combinations of thrusters. The selected thrusters are ordinarily located on opposite sides of the center of gravity. When in orbit, a spacecraft thruster may fail or may have a significant change in specific impulse characteristics. When a thruster fails, those combinations of torque for which the failed thruster is an element cannot be used. This may adversely affect or limit control of the spacecraft.
A spacecraft velocity and attitude control arrangement is desired in which a change in specific impulse of a particular thruster need not necessarily eliminate its use in controlling the spacecraft.